Hi there, Stephane Couronne.

Yes, you can add a discount by line item on invoices and sales receipts. To do this,  I suggest creating a product item for the discount. Then, enter the amount as negative.

Here's how:

1. Click the Gear icon at the upper right corner.
2. Select Products and Services under Lists.
3. Click New.
4. Select Non-inventory or Service for the type.
5. Enter the unique name of the discount.
6. Select a category. This should be the item you are selling and giving a discount.
7. In the Sales price/rate field, enter the discount as a negative amount.
8. From the Income account drop-down menu, select the Discount given account.
9. From the Sales tax category drop-down menu, select Nontaxable.
10. Hit Save and close.

Once done, follow the steps below to create invoices and sales receipts with a discount by line item.

1.  Select + New icon.
2. Choose Invoice or Sales Receipt.
3. Enter the customer's name.
4. Select the products and services. Then, add the Discount item you just created. You can adjust the discount if you need to, just make sure it's a negative amount.
5. Fill in the other nessesary field.
6. When you’re ready, select Save and send or Save and new.

You can open this article to see steps on to turn on a discount feature to use the discount field on an invoice or sales receipt and enter a specific amount: Add a discount to an invoice or sales receipt in QuickBooks Online.

You can also refer to this article to view different ways on how you can send invoices to your customers in QuickBooks Online: Send an invoice.

Please check this article on how you can record a single, partial, and joint payments in QuickBooks Online: Record invoice payments in QuickBooks Online.

Please know that you're always welcome to post if you have any other concerns. Wishing you and your business continued success.